That's a really good question. I don't have an answer, or even the beginning of an answer, but I would hazard a guess that there is a feedback loop. So listening to yourself talk (or even better, putting your thoughts down in print) is sort of like listening to someone else talk, which puts new ideas into your mind, or causes you to better organize the ones you already have.

Doing mathematical proofs might be an extreme example of that: a mathematician has (I am told) an intuition--a thought--but has to work it out rigorously. Once they've done that, the intuition becomes much clearer. I guess.